Process for producing polymeric materials

ABSTRACT

Process for the production of a homogenous polymeric monolith wherein an assembly of oriented thermoplastic polymer fibres is maintained under a contact pressure sufficient to ensure intimate contact at an elevated temperature high enough to melt a proportion of the polymer. The assembly is subsequently compressed at a compaction pressure higher than the contact pressure while still being maintained at the elevated temperature. The resulting polymeric products are useful, for example as orthodontic brackets, bone prostheses and in body armor.

This is a Rule 62 Continuation of application Ser. No. 07/934,500, filedas PCT/GB92/00401, Mar. 6, 1992 published as WO92/15440, Sep. 17, 1992,now abandoned.

This invention relates to processes for the production of polymer sheetmaterials from oriented polymer fibers and to the products of suchprocesses.

BACKGROUND OF THE INVENTION

One method which is widely used to produce high modulus polymer sheetsis the formation of fiber reinforced composites using, e.g. orientedpolyethylene fibers in order to reinforce the polymer matrix. Themanufacture of such composites is a complex operation and in particularrequires careful mixing of the polymer and the fibers if the compositeis to exhibit homogeneous mechanical properties.

There have been proposals to produce polymeric sheets by compression ofnetworks of polymer fibers at elevated temperatures most notably inrelation to thermotropic liquid crystal polymers. European Patent 354285and U.S. Pat. No. 4,384,016 both describe processes in which fibers of aliquid crystal polymer are hot pressed to produce an oriented polymersheet. European Patent Application 116845, describes a process in whicha network of fibers of ultra-high molecular weight polyethylene are hotcompressed to form polymer sheets. In the processes taught in thisdocument the fibers are compressed and heated simultaneously. Theproducts retain a significant proportion of the properties of the fibersin the direction in which the fibers are aligned but the mechanicalproperties of the products in the direction transverse to that in whichthe fibers are aligned is less than ideal. These processes arerelatively unaffected by the choice of compaction temperature. Thepolymer fibers do not melt during the process.

DESCRIPTION OF THE INVENTION

We have now discovered a novel process whereby an assembly of fibers oforiented polymer may be hot compressed to form a sheet having superiormechanical properties particularly in the direction transverse to thatIn which the fibers are aligned. The novel processes are distinguishedfrom those of EPA 116845 by an initial processing step in which thefibers are brought to and held at the compaction temperature whilstsubject to a pressure sufficient to maintain the fibers in contact, thecontact pressure, and thereafter compacted at a higher pressure, thecompaction pressure. In the processes of this invention the compactiontemperature does influence the mechanical properties of the compactedproduct. In the processes of this invention a proportion of the polymermaterial in the fibers melts and subsequently recrystallizes and it isthis melt phase which serves to bind the fibers together.

Accordingly from one aspect this invention provides a process for theproduction of a polymer sheet in which an assembly of oriented polymerfibers is maintained in intimate contact at an elevated temperaturesufficient to melt a proportion of the polymer and subsequentlycompressed so as to produce a coherent polymer sheet.

In the preferred processes of this invention the conditions and moreparticularly the temperature at which the fibers are compacted will besuch as to cause a portion of the polymer to be selectively melted. Oncooling the molten materials recrystalise to give a phase with a lowermelting point than the original fiber. The presence of a second phase inthe compacted product may readily be detected e.g. by Different ScanningCalonmetry (DSC) measurements. In general the amount of material meltedis preferably at least 5% and usually at least 10% of the original. Theapplicants believe that this minimum amount is required in order fillthe spaces between fibers upon compaction and hence produce a productwhich does not contain trapped air. Processes in which a greaterproportion of the polymer material is melted at the contact stage areuseful in so far as the mechanical properties of the product in thedirection transverse to the alignment of the fibers may be improved butthis improvement is achieved at the expense of the properties in thedirection of the alignment of the fibers. We have discovered that theimprovements in the transverse direction are not directly proportionalto the losses in the direction of alignment and that the loss is greaterthan the improvement. For most applications of the products of thisinvention the preferred processes are those which are carried out in amanner which selectively melts from 5 to 10% by weight of the polymermaterial although processes which melt from 10 to 20% by weight of thepolymer or even up to 50% by weight may be useful.

In a preferred embodiment the temperature at which the fibers arecompacted is not greater than the peak temperature of melting i.e. thetemperature of which the endotherm measured by Differential ScanningCalorimetry (DSC) of the polymer fibers reaches its highest point. Theminimum temperature at which the fibers should be contacted ispreferably that at which the leading edge of the endotherm extrapolatedto zero intersects the temperature axis.

The pressure at which the assembly of fibers is maintained during thisstage of the process will be such as to maintain the individual fibersin intimate contact but not such as will compact them and in particularnot inhibit the selective melting of the polymer. In general pressuresin the range 0.5 to 2.0 MPa are preferred. The precise value is notnormally critical.

The compaction pressure exerted upon the heated assembly of orientedpolymer fibers should be sufficient to produce a homogeneous product butshould not be so great as to cause the assembly to be extruded. Ifnecessary a closed mould may be used to prevent extrusion and thusallows the use of higher temperatures or pressures if required. Ingeneral, pressures in the range of 40 to 50 MPa have been found to beuseful. The minimum pressure required to process an assembly of aparticular polymer fiber at a particular temperature may be determinedby routine experiment.

The time required for the processes of this invention may be determinedby empirical means. The time required to bring the assembly of fibers upto the requisite temperature will vary with the nature and size of theassembly, the nature of the polymer and the heating means which areemployed. The time is not critical provided it is sufficient to enablethe selective melting to be achieved.

The time required for the compaction step is also non-critical except inso far as it must be sufficiently long to enable the assembly to becompacted. At the preferred temperatures the minimum time may be of theorder of seconds although longer times may be utilized. Processes whichutilise shorter compaction times e.g. 5 to 30 seconds may beadvantageous in so far as they may conveniently be operated upon acontinuous basis for example a uniaxially aligned assembly of heatedfibers may be passed between a pair of rollers.

The products of the processes of this invention preferably retain atleast 50% and more preferably at least 75% of the mechanical properties,especially the modulus of the oriented fibers in the direction in whichthose fibers are aligned. The products exhibit a homogeneous appearanceto the eye. Products which when stressed in the direction transverse tothat in which the fibers are aligned fibrillate, i.e. break whilstleaving the polymer fibers essentially intact are not homogeneous. Theproducts of this invention exhibit homogeneous behaviour when stressedin this transverse direction. Preferably they will be such that theattenuation of an ultrasonic C scan shows not more than a 20% variationand preferably not more than a 10% variation over the whole sample.

The assembly of oriented polymeric fibers which may be utilised in theprocesses of thls Invention may take a variety of forms. In particularthey may be arranged as an uniaxially aligned bundle or a twisted bundleof fibers or an assembly of chopped fibers or as a mat of Interwovenbundles or a mat formed by layering of bundles of fibers wherein thebundles in each layer are aligned at an angle, e.g. convenientlyperpendicular to one another. The products obtained by processing suchmats may thus retain the majority of the properties of the orientedfibers in more than one direction. The bundles may be assembled andpressed into any convenient shape. The products may be flat sheets,rods, bars, any of which may be shaped so as to be suitable forparticular applications.

The oriented polymer fibers may be obtained by any of the knownmanufacturing processes. In particular, fibers which have been producedby melt spinning and drawing and gel spinning and drawtng. Typicallysuch fibers will have a diameter in the range 0.005 to 0.05 mm.

The processes of this invention may be carried out using conventionalequipment. Conveniently, the fiber assembly may be placed in a suitablemould and placed under contact pressure. The assembly may then bepreheated to the desired temperature at such a rate as to ensure thatthere is no significant temperature gradient across the assembly. Thedesired compaction pressure is then applied and maintained forsufficiently long for the fibers to cohere. The hot compacted materialsare preferably cooled to ambient temperature under controlledconditions. Rapid cooling is less preferred. The most convenienttechnique is to allow the compacts to stand in the air until they havecooled to ambient temperature.

The processes of the present invention may utilise any polymer fiberswhich can be selectively melted. The susceptibility of particularpolymers and particular grades of that polymer to selective meltingvaries and their suitability for use in the processes of this inventionmay be determined empirically.

The processes of the present invention find particular application inthe production of oriented polyolefin articles especially orientedpolyethylene articles. The polyethylene (which may be a homo orcopolymer of polyethylene) may have a weight average molecular weight Mwof from 50,000 to 3,000,000. For polyethylene articles the temperatureto which the assembly is preheated is preferably within 5° C. and morepreferably within 2° C. of the peak temperature of melting. Orientedpolyethylene products of the processes of this invention preferably havea transverse (i.e. in the direction perpendicular to that in which thefibers are aligned) strength of at least 15 MPa and more preferably atleast 25 MPa.

Gel spun polyethylenes having a weight average molecular weight of atleast 500,000 may exhibit extremely high axial tensile modulus. Thiscorresponds to an extremely high degree of alignment of the polymermolecules within the fibers. These highly oriented gel spun materialsmay be processed according to this invention and may be preferred whereit is desired to produce a product which exhibits high strength in thedirection of the fiber alignment. However the strength in the directiontransverse to this alignment may be limited unless relatively highproportion of the axial strength is sacrificed by allowing the polymerto melt. Polymer fibers which are not so highly oriented may bepreferable in so far as the selective melting which characterizes theprocesses of this invention may affect the axial properties to a lesserdegree whilst producing useful strengths in the transverse direction.

Homo and co polymers of polyethylene having a weight average molecularweight of from 50,000 to 500,000 particularly those which can beproduced by melt-spinning from a preferred raw material for use in theprocesses of this invention. Such polymers appear to be more amenable tothe selective melting process either by virtue of their comprising somepolymer having a relatively low molecular weight or by virtue of theirhaving a surface layer which melts at a lower temperature. Whatever themechanism which is involved those polymers are preferred because theycan form compacts which retain a large proportion of the properties ofthe fiber (in the direction of alignment of that fiber) whilst producingproducts having superior properties in the direction transverse to thatalignment.

Other classes of polymer fibers which may be useful in the processes ofthis invention include any of the known orientable polymers. Inparticular the oriented polymer may be an unsubstituted or mono or polyhalo substituted vinyl polymer, an unsubstituted or hydroxy substitutedpolyester, a polyamide, a polyetherketone or a polyacetal. Suitableexamples include vinyl chloride polymers, vinyl fluoride or vinylidenefluoride polymers PHB, PEEK and homo and copolymers of polyoxymethylene.Particular examples of polyesters useful in the processes of thisinvention include those derivable by the reaction of at least onepolyhydric alcohol, e.g. a linear polyhydric alcohol preferably a diolwith at least one poly basic acid, suitably a polycarboxylic acid. Thealcohol is preferably an alicyclic or aliphaiic alcohol such ascyclohexane-dimethanol or a linear alkylene diol such as ethyleneglycol, 1,3 propylene glycol or 1,4 butylene glycol. The preferred acidsinclude o, m or ter phthalic acids, 2,6 and 1,5 napthalene dicarboxylicacid and 1,2 dihydroxy benzoic acid.

The compacted products of the present invention normally have a densityless than that of the original fiber. This reduction is caused primarilyby the retention of air within the compacted material but also by anyreduction in the content of crystalline material within the polymercaused by any molten polymer cooling to form an amorphous phase. Boththese factors detract from the properties of the product and thepreferred processes of this invention produce products in which thedensity is at least 90% more preferably at least 95% and most preferablysubstantially the same as that of the polymer fiber. This reflects thefact that the compaction should preferably be carried out in a mannerwhich expels any trapped air from the product and that in the morepreferred embodiment the compact will be cooled in a manner whichresults in the molten material forming a crystalline phase on cooling.

The processes of this invention enable complicated and precisely shapedpolymeric articles having high stiffness and high strength to bemanufactured. The products may also exhibit good energy absorbingproperties. The products find use in a wide variety of applications,particular examples being as orthodontic brackets, as bone implants andas high impact energy absorbing materials, e.g. in body armour.

The invention is illustrated by the following examples:

The tests used in these examples are defined as follows:

The fiber modulus and strength were measured on a 20 cm long sample at adisplacement rate of 20 cm/min.

The flexure modulus of the samples produced from the process weremeasured under the guidelines of ASTM D790.

The flexure strengths of the samples produced from the process weremeasured under the guidelines of ASTM D790.

The short beam shear strength of the samples measured under theguidelines of ASTM D2344.

The densities of the compacted materials were measured using a densitybottle.

Ultrasonic elastic properties were measured using an immersion method ata frequency of 2.25 MHz. A full description of the technique can befound in S. R. A. Dyer, D. Lord, I. J. Hutchinson, I. M. Ward and R. A.Duckett, J. Phys. D:Apply. Phys. 25 (1992) 66.

The fibers used were polyethylene fibers having the followingparticulars:

    __________________________________________________________________________                                    Tensile                                                   Molecular      Breaking                                                                           initial                                                                           modulus                                               Weight         Strength                                                                           secant                                                                            2%                                        Sample                                                                            Fibre   Mw    Mn  Process                                                                            GPa  GPa GPa                                       __________________________________________________________________________    1   CELANESE                                                                               61,000                                                                             28,000                                                                            melt 1.0  54  36                                                              spun                                                    2   SNIA FIBER                                                                            130,000                                                                             12,000                                                                            melt 1.3  58  43                                                              spun                                                    3   TEKMILON                                                                              700,000                                                                             54,000                                                                            solvent                                                                            2.1  80  70                                                              spun                                                    4   SPECTRA 1,500,000                                                                           75,000                                                                            gel  2.9  130 115                                           1000              spun                                                    __________________________________________________________________________

EXAMPLES

The invention will no be described in more detail with reference to thefollowing working examples.

Example 1

A sheet of dimensions 3 mm×5 cm×10 cm was prepared by hot pressing aunidirectionally aligned bundle of melt spun SNIA high moduluspolyethylene fibers having a diameter of 0.015 mm in an open endedmatched metal mould. The fibers were preheated for 10 minutes undercontact pressure of 0.5 MPa at 139°±0.5° C. and then a pressure 400 MPawas applied for 10 seconds. The resulting product was a homogeneoustranslucent sheet with the following properties.

    ______________________________________                                        Tensile modulus in fiber direction                                                               57     GPa     measured                                    Tranverse to fiber direction                                                                     4.2    GPa     ultrasonically                              Flexure modulus in fiber direction                                                               35     GPa     ASTM D790                                   Tranverse to fiber direction                                                                     3.2    GPa                                                 Short beam shear strength                                                                        29     GPa     ASTM D2344                                  Flexure strength in fiber                                                                        110    MPa     ASTM D790                                   direction                                                                     Transverse to fiber direction                                                                    31     MPa                                                 ______________________________________                                    

An ultrasonic immersion `C` scan of the product showed only a 2% changein attenuation over the sample and is taken as a measure of thehomogeneity of the product.

A DSC trace of the compacted material showed that 8% of the originalfiber phase had been melted and had recrystallized forming a secondlower melting point phase.

The density of the compacted material was 90% of the original fiberdensity.

Example 2

A bar of 3 mm square cross section was prepared by hot pressing atwisted bundle of melt spun SNIA high modulus polyethylene fibers havinga diameter of 0.015 mm in an open ended matched metal mould. The fiberswere preheated at 139°±0.5° C. for 10 minutes and then pressed for 30seconds at a pressure of 50 MPa. The resulting product was a homogeneoustranslucent bar with a flexural modulus (ASTM D790) of 32 GPa.

Example 3

An orthotropic material was made by compacting a number of layers of awoven mat of melt spun SNIA high modulus polyethylene fibers in an openended matched metal mould. The laminated mat was maintained at 139°±0.5°C. for 10 minutes at 0.5 MPa before applying a high pressure of 50 MPafor 30 seconds. The flexure modulus was the same in both the axes in theplane of the plate, with a value of 11 GPa. The flexure strength wasalso similar in the two axes in the plane of the plate with a value of85 MPa. We can conclude that using a woven mat for compaction results ina substantial improvement in transverse strength at the expense ofstiffness.

Example 4

A three dimensional shape was formed by compacting a number of layers ofa woven mat of melt spun SNIA high modulus polyethylene fiber betweenmale and female hemispherical moulds. The compaction conditions wereidentical to those shown in example 3. The compacted material was formedinto the required shape in a single process.

Example 5

A laminated sheet 3 mm thick and 55 mm square was made by sandwiching auniaxially aligned bundle of melt spun SNIA polyethylene fibers betweentwo layers of a woven mat of melt spun SNIA polyethylene fibers. Thesandwich was then compacted using conditions given in example 3. Theresult was a translucent sheet with the following properties.

    ______________________________________                                        Tensile modulus in fiber direction                                                               52     GPa    measured                                     Tranverse to main fiber direction                                                                4.9    GPa    ultrasonically                               Flexure modulus in main fiber                                                                    18     GPa    ASTM D790                                    direction                                                                     Flexure strength transverse to                                                                   75     MPa    ASTM D790                                    main fiber direction                                                          ______________________________________                                    

Lamination allows a better compromise to be achieve between stiffnessand strength, especially in tension.

Example 6

2.0 grams of chopped melt spun SNIA high modulus polyethylene fiber wasplaced in a cylindrical mould which was 12 mm in diameter and 30 mmlong. Compaction of the fiber assembly proceeded according to theconditions described in example 3. The resulting cylindrical bar was anisotropic material having a modulus of 5 GPa. A DSC trace of the productshowed that 12% of the original fiber had been melted.

Example 7

A bar of 25 mm square cross section and 100 mm long was prepared by hotpressing a number of cold compacted layers of melt spun SNIA highmodulus polyethylene fibers in a closed matched metal mould usingconditions described in example 3. DSC traces taken through thecompacted blocks showed that a reasonably even heat distribution hadbeen achieved.

Example 8

3.0 grams of melt spun CELANESE high modulus polyethylene fiber with adiameter of 0.015 mm was compacted in an open ended rectangular sectionsteel mould at a compaction temperature of 134°±0.5° C. A contactpressure of 0.5 MPa was held for 10 minutes and then a pressure of 40MPa was applied for 30 seconds. The sample had the appearance of a solidpolyethylene rod with a well defined cross section measuring 3.34mm×3.11 mm. The bending modulus was 19.7 GPa.

Example 9

To demonstrate the criticality of the moulding temperature, a sampleidentical to that used in example 8 was compressed in the same mould atthe higher temperature of 138° C. The resulting sample again had theappearance of a solid polyethylene rod but the low bending modulus of1.2 GPa showed that the properties of the fiber had been lost due tosubstantial melting of the original fiber phase. Further evidence of thecritical nature of the temperature was shown by compressing an identicalsample to examples 8 and 9 but at the lower temperature of 127° C. Theresulting product had a high stiffness but poor transverse propertiesdue to almost total retention of the original fiber phase.

Example 10

The role of pressure was examined by carrying out an identicalexperiment to example 1 except that high pressure (40 MPa) was appliedfrom the very start of the procedure, including the warm up period. Theresulting product had a high longitudinal stiffness of 60 GPa but a poortransverse strength of 12 MPa. A DSC trace of the compacted materialshowed no evidence of any `second phase`: the compacted material wascomposed entirely of the original fiber phase.

We can therefore conclude that applying high pressure from the beginningof the compaction process inhibits the selective melting which isnecessary for optimum control of the properties of the final product.

Example 11

A sheet of dimensions 3 mm×55 mm×55 mm was prepared by compacting aunidirectionally aligned bundle of gel spun SPECTRA high moduluspolyethylene fibers in a matched metal mould. The processing conditionswere identical to example 3 apart from raising the compactiontemperature to 152°±0.5° C., which is midway between the onset ofmelting and the end of melting.

The resulting compacted sheet was homogeneous and had a longitudinalmodulus of 35 GPa and a transverse strength of 17 MPa. A DSC trace ofthe compacted material showed around 35% of a `second phase` formed bymelting of the original fiber.

We claim:
 1. A process for the production of a homogeneous polymericmonolith from thermoplastic polymer fibers, the monolith having improvedmechanical properties in a direction transverse to said fibers, saidprocess comprising the steps of:forming an assembly of orientedthermoplastic polymer fibers; applying a contact pressure to saidassembly of oriented thermoplastic polymer fibers sufficient to ensureintimate contact of the polymer fibers with each other, heating saidassembly of polymer fibers to an elevated temperature sufficient toselectively melt a proportion of the polymer fibers while in intimatecontact with each other, the molten polymer on cooling recrystallizingto form a melt phase which has a melting point less than the meltingpoint of the fibers and which binds said fibers together; andsubsequently compressing said bound fibers at a compaction pressurehigher than said contact pressure while still maintaining said assemblyat an elevated temperature to produce said monolith.
 2. A processaccording to claim 1, wherein said assembly is maintained at atemperature at least that at which an extrapolation of a leading edge ofan endotherm of the oriented fibers measured by differential scanningcalorimetry intersects a temperature axis.
 3. A process according toclaim 1, wherein the assembly is maintained at a temperature such that 5to 20% by weight of the polymer fibers is melted.
 4. A process accordingto claim 3, wherein the assembly is maintained at a temperature suchthat from 5 to 10% weight of the polymer fibers is melted.
 5. A processaccording to claim 1, wherein the heated assembly is compressed under acontact pressure of from 0.5 to 2.0 MPa.
 6. A process according to claim5, wherein the contact pressure is maintained for a period of 5 to 30seconds.
 7. A process according to claim 1, wherein said compressingproduces a hot compressed material which is allowed to cool to ambienttemperature by standing in air.
 8. A process according to claim 1,wherein the polymer fibers are selected from tile group consisting ofhomo and copolymers of a polyolefin.
 9. A process according to claim 8,wherein the polymer fibers are polyethylene fibers.
 10. A processaccording to claim 8, wherein the polymer fibers have a weight averagemolecular weight of from 50,000 to 3,000,000.
 11. A process according toclaim 8, wherein the polymer fibers have a weight average molecularweight of from 500,000 to 3,000,000.
 12. A process according to claim 8,wherein the polymer fibers have a weight average molecular weight offrom 50,000 to 300,000.
 13. A process according to claim 8, wherein thefiber is a melt spun fiber.
 14. A process according to claim 8, whereinthe assembly is maintained at a temperature which is no more than 5° C.below a peak temperature of melting of the polymer fibers.
 15. A processaccording to claim 8, wherein the assembly is maintained at atemperature which is no more than 2° C. below a peak temperature ofmelting of the polymer fibers.
 16. A process according to claim 1,wherein the compressed product has a density which is at least 90% ofthe original fiber density.
 17. A process according to claim 1, whereinthe polymer fibers are fibers of a polymer selected from the groupconsisting of a vinyl polymer, a polyester, a polyamide, apolyetherketone and a polyacetal.
 18. A process according to claim 7,wherein said polyester is polyethylene terephthalate.
 19. A processaccording to claim 1, wherein said compaction pressure is in the rangefrom 40 to 400 MPa.
 20. A process according to claim 19, wherein saidcompaction pressure is in the range of 40 to 50 MPa.
 21. A processaccording to claim 1, wherein said contact pressure is applied for about10 minutes.
 22. A process according to claim 21, wherein said compactionpressure is applied for 5 to 30 seconds.
 23. A process for theproduction of homogeneous polymeric monolith from thermoplastic polymerfibers, the monolith having improved mechanical properties in adirection transverse to said fibers, said process comprising the stepsof:forming an assembly of oriented thermoplastic polymer fibers;applying a contact pressure to said assembly of oriented thermoplasticpolymer fibers to ensure intimate contact of the polymer fibers witheach other, heating said assembly of polymer fibers to a temperature notgreater than the temperature of which the endotherm measured byDifferential Scanning Calorimetry of the polymer fibers reaches itshighest point, to selectively melt a proportion of the polymer fiberswhile in intimate contact with each other, the molten polymer on coolingrecrystallizing to form a melt phase having a melting point less thanthe melting point of the fibers and which binds said fibers together;and subsequently compressing said bound fibers at a compaction pressurehigher than said contact pressure while still maintaining said assemblyat an elevated temperature to produce said monolith.
 24. A processaccording to claim 23, wherein said contact pressure is from 0.5 to 2.0MPa.
 25. A process according to claim 23, wherein said compactionpressure is from 40 to 400 MPa.
 26. A process according to claim 23,wherein said contact pressure is applied for about 10 minutes.
 27. Aprocess according to claim 23, wherein said compaction pressure isapplied for 5 to 30 seconds.
 28. A process for the production ofhomogeneous polymeric monolith from thermoplastic polymer fibers, themonolith having improved mechanical properties in a direction transverseto said fibers, said process comprising the steps of:forming an assemblyof oriented thermoplastic polymer fibers; applying a contact pressure tosaid assembly of oriented thermoplastic polymer fibers sufficient toensure intimate contact of the polymer fibers with each other, heatingsaid assembly of polymer fibers to an temperature sufficient toselectively melt no more than 20% by weight of the polymer fibers whilein intimate contact with each other, the molten polymer on coolingrecrystallizing to form a melt phase which has a melting point less thanthe melting point of the fibers and which binds said fibers together;and subsequently compressing said bound fibers at a compaction pressurehigher than said contact pressure while still maintaining said assemblyat an elevated temperature to produce said monolith.
 29. A processaccording to claim 28, wherein from 5 to 10% by weight of the polymerfibers are selectively melted.
 30. A process according to claim 28,wherein said contact pressure is from 0.5 to 2.0 MPa.
 31. A processaccording to claim 28, wherein said compaction pressure is from 40 to400 MPa.
 32. A process according to claim 28, wherein said contactpressure is applied for about 10 minutes.
 33. A process according toclaim 28, wherein said compaction pressure is applied for 5 to 30seconds.